The adjustment of the tension of a material web is always of importance when the material web must be processed exactly. A typical example is an electrographic printing device in which a recording medium is printed as a material web in a printing group. Such electrographic print devices are presumed in the following discussion.
In such an electrographic print device, a clean/faultless print image is only created when the material web (e.g., a paper web) is supplied to the printing group of the print device in a taut, pre-stressed state. For this, it is known from German Patent Document No. DE 197 49 603 C2 to provide a loop-puller device via which the material web is guided. The loop-puller device comprises a loop-puller roller, under spring tension and movable in a predetermined region, via which the material web is stressed.
In normal operation, the deflection of the loop-puller roller is sufficient to keep the material web stressed. However, there are situations in which additional measures are necessary in order to keep the material web in a stressed state. For these cases, the position of the loop puller roller is scanned and, dependent on its position, a control signal is generated with which a braking device is triggered that brakes the material web such that the tension of the material web is maintained. As a braking device, a negative pressure brake is used that pulls on the material web dependent on the control signal and thus brakes it. This is described in German Patent Document No. DE 197 49 603 C2.
In operation, the transport of the material web should be designed such that, for example, the material web optimally, rapidly reaches its desired position upon starting. The braking device should be designed such that various widths of the material web, material webs with marginal perforations, etc. can be used. However, there are problems with the use of a negative pressure brake as a braking device when, for example, the material web is corrugated or comprises holes (material webs with marginal perforations). However, a braking device should function with material webs of various widths, material webs with marginal perforations and such with cut-outs. The braking force should thereby be able to be adjusted quickly.
For this, in addition to the web tension, the position of the material web must also be maintained. This is necessary in order to achieve consistent processing results. Due to inhomogeneities of the material web or non-continuous interferences via the processing process, a regulation of the web tension and the web position is mostly necessary. What is particularly problematic is mutual influencing of the parameters, i.e., upon change of the web position, a change of the web tension or the web tension distribution over the width of the material web results.
To adjust the web position, a rotating frame can be used with which the material web can be deviated multiple times. The material web can thereby be displaced depending on the relative position of the deflections. A device to deflect a material web is, for example, known from German Patent Document No. DE 198 27 254 A1.